Field of the Invention
An electromagnetic boat speedometer includes a boundary layer velocity compensating arrangement comprising a primary coil for producing a primary electromagnetic field within a relatively large first volume of water, a secondary coil for producing a secondary electromagnetic field within a relatively small portion of the first volume of water immediately adjacent the hull of the boat, and a set of first electrodes removably mounted in one or more openings in the hull of the boat, such that the tips of the first electrodes extending into the relatively small water portion. The coils are energized simultaneously in opposition, or alternately.
Description of Related Art
Electromagnetic flowmeters for measuring the flow of fluid in pipes are well known in the patented prior art, as shown by the inventor's Marsh U.S. Pat. No. 6,598,487, and the patent to Gardner U.S. Pat. No. 4,079,626.
In the boating industry, most pleasure craft utilize some sort of speed indicating device to determine the boat's speed relative to the water. Typical of such devices are paddle wheels, impellors, pitot tubes, ultrasonic sensors and electromagnetic sensors.
Paddle wheels and pitot tubes are prone to repeated fouling by marine growth as well as by debris in the water. Ultrasonic sensors and EM sensors are generally less prone to foul. Consequently, speed sensors are generally designed so that they can be removed from the boat for cleaning, even when the boat is in the water.
In order to facilitate the removal process, manufacturers typically supply the boat owner with a “sea valve” or a special thru-hull penetrator that allows the boat owner to insert various sensors that must make contact with the water to properly function. Typically these sensors include “O-rings” that prevent water from leaking into the boat through this hull penetration.
A problem associated with most existing boat speedometers is that the speed measurement is made very close to the hull of the boat and in such a small volume of water that the speed measurement can be adversely effected by the boat's hull, especially at high speeds and on larger boats.